The invention relates to a method and apparatus for producing objects of glass by using the so-called levitating method. A preform made of glass or glass ceramics is shaped by means of a shaping tool which is arranged as a porous membrane and comprises a gas connection. A gas cushion is built up by means of the gas connection between the surface of the levitation mold facing the preform and the preform per se. This avoids any direct contact between the preform and the levitation mold. This leads to numerous advantages. The gluing of the glass on the mold surface of the levitation mold is thus prevented, among other things.
EP 070 760 A shows and describes a method as well as an apparatus for producing a formed body made of glass. A gob of the hot melt is transferred to the mold The mold is arranged as a levitation mold The walls of the mold are thus porous. A pressure gas is connected to the levitation mold which emerges from the porous of the levitation mold below the preform and thus prevents any contact between the same and the shaping surface.
In the case of such a contactless shaping of gobs of molten glass by means of gas levitation the most homogeneous temperature in the glass is an important prerequisite for a perfect quality of the finished product. This is the case in particular when high-quality glasses are produced, e.g. for optical applications.
This goal is frequently not achieved The temperature distribution in the glass is in no way always homogeneous. This often caused by the fact that the levitation mold absorbs a part of the heat contained in the gob (especially by radiation) and that this heat absorption occurs in an uncontrolled fashion. The differences can occur from gob to gob, influenced by the different masses of these gobs. Differences in heat absorption can also occur within the gob due to the fact that the levitation mold is thicker at some places and thinner at others, which is the result of the shape of the levitation mold which is required for the shaping.